The present invention relates to an aberration detector which detects a spherical aberration appearing in a light-gathering optical system and further concerns an optical pickup device which is provided with the aberration detector.
Generally, in order to increase a recording density in an optical disk device, it is necessary to minimize a wavelength of light used for recording and reproducing in an optical disk, which serves as a recording medium, or it is necessary to set a large numerical aperture (NA) for an object lens, which allows light to converge on the optical disk.
In order to reduce a wavelength of light, it is necessary to develop a semiconductor laser for generating a laser beam having a shorter wavelength. However, such a semiconductor laser cannot be readily developed, so that a method for increasing the numerical aperture of the object lens has been conventionally adopted to realize a higher recording density.
In order to increase the numerical aperture of the object lens, a method for increasing a diameter of the lens can be adopted. However, in this case, a large device is necessary and other problems occur. For this reason, a method has been advised, in which to apertures of the object lens are effectively increased by using a solid immersion lens without increasing a diameter of the objective lens.
For instance, Japanese Laid-Open Patent Application 8-212579/1996 (Tokukaihei 8-212579, published on Aug. 20, 1996) discloses an optical pickup device using the solid immersion lens. As shown in FIG. 6, in the optical pickup device, light gathered by an object lens 112 is transmitted through a substrate lllb of a magneto-optical disk 111 via a plate 113 and a solid immersion lens 114, the light is gathered on the an information recording layer 111a, and then, a magnetic head 115 records information. The magnetic head 115 is disposed such that the magneto-optical disk 111 is placed between the magnetic head 115 and the solid immersion lens 114.
The edge of the object lens 112 is held by a holder 118. The outer edge of the holder 118 is provided with a focusing actuator 119 for controlling a focus of the object lens 112, and a tracking actuator 120 for controlling tracking.
Meanwhile, the edge of the solid immersion lens 114 is held by a holder 116. The outer edge of the holder 116 is provided with a solid immersion lens actuator 117 for adjusting a space between the solid immersion lens 114 and the plate 113 or the object lens 112.
Here, the solid immersion lens 114 is made of glass having virtually the same refractive index as the substrate 111b of the magneto-optical disk 111, and a hemispheric surface is a spherical surface having a light-gathering point at the center. Thus, regarding light gathered by the object lens 112, the numerical aperture is multiplied by the refractive index in the substrate 111b. Specifically, when the numerical aperture is 0.55 in the objective lens 112 and the refractive index of the solid immersion lens 114 is 1.5, an effective numerical aperture is 0.83.
As described above, a light-gathering optical system using the solid immersion lens 114 has a large effective numerical aperture. However, a large spherical aberration appears due to a thickness error occurring in the substrate 111b of the magneto-optical disk 111 and due to a change in a thickness of the substrate 111b in the case of a multilayer structure.
Therefore, when a spherical aberration appears in the above light-gathering optical system including the solid immersion lens 114 and the object lens 112, the solid immersion lens actuator 117 is used for adjusting a space between the solid immersion lens 114 and the plate 113 or the object lens 112, so that the spherical aberration is corrected.
Specifically, the holder 116 and the holder 118 are respectively provided with electrodes which oppose each other, and electrical capacity is measured between the electrodes. At this time, the solid immersion actuator 117 shifts the holder 116 toward the holder 118 and maintains a space between the holders 116 and 118 such that the electrical capacity is at a predetermined value. Thus, the spherical aberration is artificially corrected in the light-gathering optical system.
Incidentally, in the above optical pickup device, a space between the holders 116 and 118 is maintained such that electrical capacity between the holders 116 and 118 is at a predetermined value. Hence, a spherical aberration is corrected in the light-gathering optical system.
Therefore, in the above optical pickup device, the electrical capacity is measured so as to detect the spherical aberration of the light-gathering optical system.
However, the electrical capacity measured between the holders 116 and 118 is an extremely small value of no more than 10 pF, so that an error may appear due to stray capacitances caused by wires and others in the optical pickup device. In this case, it is not possible to accurately detect a spherical aberration in the light-gathering optical system.
As mentioned above, when it is not possible to accurately detect a spherical aberration appearing in the light-gathering optical system, the appearing spherical aberration cannot be corrected in an appropriate manner. As a result, it is not possible to suitably record and reproduce information in the information recording layer 111a of the magneto-optical disk 111.
The object of the present invention is to provide an aberration detector which can accurately detect a spherical aberration appearing in the light-gathering optical system, without being affected by an ambient electrical noise, and to provide an optical pickup device which includes the aberration detector so as to suitably correct a spherical aberration appearing in a light-gathering optical system and which can suitably record and reproduce information in a magneto-optical disk.
In order to achieve the above object, the aberration detector of the present invention is provided with a detecting section for detecting a spherical aberration of the light-gathering optical system in accordance with two focus positions of a first light beam passing near an optical axis and a second light beam passing outside the first light beam, among light beams passing through the light-gathering optical system.
Therefore, the detecting section divides light beams passing through the light-gathering optical system into the first light beam passing near the optical axis and the second light beam passing outside the first light beam, and the detecting section detects a spherical aberration of the light-gathering optical system in accordance with the focus positions of the light beams. With this arrangement, it is possible to optically detect a spherical aberration appearing in the light-gathering optical system.
Hence, unlike a conventional device which electrically detects a spherical aberration appearing in the light-gathering optical system, a spherical aberration can be precisely detected without being affected by an ambient electrical noise.
As mentioned above, when a spherical aberration of the light-gathering optical system can be precisely detected, it is possible to correct the spherical aberration of the light-gathering optical system in an appropriate manner.
Further, the optical pickup device of the present invention is provided with a light source; a light-gathering optical system for gathering light, which is emitted from the light source, on a recording medium; a detecting section for detecting a spherical aberration of the light-gathering optical system in accordance with two focus positions of a first light beam passing near an optical axis and a second light beam passing outside the first light beam; and an aberration correcting section for correcting the spherical aberration of the light-gathering optical system in accordance with the output of the detecting section.
Thus, the detecting section divides light beams passing through the light-gathering optical system into the first light beam passing near the optical axis and the second light beam passing outside the first light beam, and the detecting section detects a spherical aberration of the light-gathering optical system in accordance with the focus position of the light beams; subsequently, the spherical aberration can be detected optically.
Therefore, unlike a conventional device which electrically detects a spherical aberration appearing in the light-gathering optical system, a spherical aberration can be precisely detected without being affected by an ambient electrical noise.
Further, the aberration correcting section can correct a spherical aberration of the light-gathering optical system in accordance with a spherical aberration precisely detected by the detecting means. Consequently, it is possible to suitably record and reproduce information in an optical recording medium.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.